75As and 139La NMR studies on LaFeAs(O1-xFx)

Abstract

We report experimental results of 75As and 139La nuclear magnetic resonance (NMR) in the layered oxypnictide system LaFeAs(O1-xFx) (x=0.0, 0.04, 0.07 and 0.11) where superconductivity occurs in x greater than 0.04. In the undoped LaFeAsO, 1/T1 of 139La exhibits a distinct peak at TN~142K below which the La NMR spectra become broadened due to the internal magnetic field attributed to an antiferromagnetic (AFM) ordering. In the x=0.04 sample, 1/T1T of 75As exhibits a Curie–Weiss temperature dependence down to 30 K, suggesting the development of AFM spin fluctuations with decreasing temperature. In the x=0.11 sample, in contrast, pseudogap behavior is observed in 1/T1T at the 75As site with a gap value of ΔPG~175K. The spin dynamics vary markedly with F-doping, which is ascribed to the change of the Fermi-surface structure by the electron-doping. As for the superconducting properties for x=0.04, 0.07 and 0.11, 1/T1 of 75As in all compounds does not exhibit a coherence peak just below Tc and follows a T3 dependence at low temperatures. These results seemingly suggests that unconventional superconductivity with zero gap along lines, whereas the lack of the residual density of states at the low temperatures is incompatible with the presence of the line-nodes. We discuss similarity and difference between LaFeAs(O1-xFx) and cuprates, and also discuss the relationship between spin dynamics and superconductivity on the basis of F-doping dependence of Tc and 1/T1.